1. Technical Field
The present disclosure relates to a laser gas analyzer. In particular, the present disclosure relates to dynamic auto gain control of a received optical signal.
2. Related Art
A laser gas analyzer using the tunable diode laser absorption spectroscopy (TDLAS) method has the following advantages. That is, the analyzer can measure the concentration of a high-temperature component to be measured or a component to be measured containing a corrosive gas or the like only by irradiating the component to be measured with light from a tunable diode laser with high component selectivity, in a non-contact manner, at high speed, and in real time without being subject to interference of other components.
FIG. 2 is a block diagram exemplifying a laser gas analyzer in the related art using the TDLAS method. The analyzer includes a light source unit and a detection unit. The light source unit contains a diode laser. The diode laser irradiates an atmosphere of a gas to be measured with measuring laser light. The detection unit contains a light receiving element and an arithmetic processing unit. The light receiving element detects the measuring laser light that has passed through the measuring space of the atmosphere of the gas to be measured. The arithmetic processing unit processes an output signal of the light receiving element.
The laser gas analyzer shown in FIG. 2 measures an inherent light absorption spectrum of molecules, which are components to be measured present ranging from an infrared region to a near infrared region, by using a diode laser in which the emission wavelength spectral line width is extremely narrow. The molecule-inherent light absorption spectrum corresponds to a molecule vibration or a rotation energy transition. Inherent absorption spectra of many molecules including O2, NH3, H2O, CO, and CO2 are in the infrared to near infrared regions. The concentration of the target component can be calculated by measuring the absorbed amount (absorbance) of light at a specific wavelength.
As shown in FIG. 2, a diode laser 11 is provided in a light source unit 10. The diode laser 11 irradiates an atmosphere of a gas to be measured 20 with measuring laser light. The line width of the emission wavelength spectrum of the laser light irradiated by the diode laser 11 is extremely narrow. The emission wavelength can be changed only by changing the laser temperature or the drive current. Thus, any one absorption peak in the absorption spectrum can be measured.
Therefore, the absorption peak can be selected by the laser gas analyzer without being affected by an interfering gas. Moreover, the analyzer has high wavelength selectivity and is not affected by other interfering components. Therefore, the analyzer can directly measure a process gas without removing the interfering gas in a stage prior to measurement.
The exact spectrum that does not overlap with interfering components can be measured by scanning the emission wavelength of the diode laser 11 near one absorption line of the component to be measured. The shape of the spectrum changes due to the broadening phenomenon of the spectrum caused by variations (environmental variations) of the temperature of the gas to be measured, the pressure of the gas to be measured, and coexisting gas components. Such environmental variations are involved in actual process measurement and thus compensation is necessary.
Thus, the analyzer in FIG. 2 uses the spectral area method. According to the spectral area method, the absorption spectrum is measured while the emission wavelength of the diode laser 11 is scanned. The spectral area is thereby determined. The component concentration is calculated based on the spectral area.
Other laser gas analyzers use the peak height method, 2f method or the like. According to the peak height method, the concentration of a component to be measured is determined from the peak height of an absorption spectrum. According to the 2f method, a wavelength signal for scanning is modulated to obtain a modulated waveform having a frequency twice the frequency of the wavelength signal. Then, the concentration of a component to be measured is determined based on a P-P (peak to peak) value of the modulated waveform. These methods are likely to be affected by variations of the temperature, pressure, or coexisting gas components.
In contrast, the spectral area method is not in principle affected by differences of coexisting gas components (the spectral area is almost the same regardless of coexisting gas components). Even if the pressure is varied, the spectral area in principle changes linearly.
According to the peak height method and 2f method, all three variable factors (the temperature, pressure, or coexisting gas components) nonlinearly affect measured values. If these variable factors coexist, it is difficult to make compensation. According to the spectral area method, on the other hand, linear compensation can be made for gas pressure variations and nonlinear compensation can be made for gas temperature variations. Therefore, accurate compensation can be realized.
The measuring laser light having passed through the atmosphere of the gas to be measured 20 is received by a light receiving element 31 provided in a detection unit 30. The light receiving element 31 converts the received laser light into an electric signal.
An output signal from the light receiving element 31 is adjusted by a gain-variable amplifier 32 so that the signal has an appropriate amplitude level. Subsequently, the output signal is input into an A/D converter 33. The A/D converter 33 converts the signal into a digital signal.
Output data from the A/D converter 33 is added up by an integrating meter 34 before being stored in a memory 35. The additions and storages are repeated in synchronization with scans of the wavelength of the diode laser 11 a predetermined number of times (for example, a few hundred to a few thousand times). Accordingly, noise contained in a measurement signal (output data) is removed. As a result, the output data is smoothed. The smoothed output data is input into a CPU 36.
The CPU 36 performs arithmetic processing such as the concentration analysis of the gas to be measured based on the measurement signal from which noise has been removed. Further, the CPU 36 adjusts the gain of the amplifier 32 if the amplitude level of the output signal of the light receiving element 31 is not appropriate as the level of a signal input into the A/D converter 33.
In the literature “Laser Gas Analyzer TDLS200 and Its Application to Industrial Processes by Kazuto Tamura and three others, Yokogawa Technical Report, Yokogawa Electric Corporation, 2010, Vol. 53, No. 2 (2010), pp. 51-54”, the principle of measurement, features, and concrete measurement examples of the laser gas analyzer in which tunable diode laser absorption spectroscopy is applied are described. This document is incorporated herein in its entirety.